This application generally relates to induced nuclear reactions, including systems, processes and elements which implement such processes, such as a reactor core, primary heat exchanger, or pump, immersed in a liquid coolant in a vessel and more particularly relates to a heat exchanger, methods therefor and a nuclear fission reactor system.
It is known that, in an operating nuclear fission reactor, neutrons of a known energy are absorbed by nuclides having a high atomic mass. The resulting compound nucleus separates into fission products that include two lower atomic mass fission fragments and also decay products. Nuclides known to undergo such fission by neutrons of all energies include uranium-233, uranium-235 and plutonium-239, which are fissile nuclides. For example, thermal neutrons having a kinetic energy of 0.0253 eV (electron volts) can be used to fission U-235 nuclei. Thorium-232 and uranium-238, which are fertile nuclides, will not undergo induced fission, except with fast neutrons that have a kinetic energy of at least 1 MeV (million electron volts). The total kinetic energy released from each fission event is about 200 MeV. This kinetic energy is transformed into heat.
In nuclear reactors, the afore-mentioned fissile and/or fertile material is typically housed in a plurality of closely packed together fuel assemblies, which define a nuclear reactor core. The fissile and/or fertile material may be a mixture of oxides of plutonium and uranium in the form of fuel pellets housed in fuel rods spaced apart by spacer or wire wound helically around each fuel rod
In addition, in a commercial nuclear power reactor, the fission heat is converted into electricity. In this regard, reactor primary coolant is pumped through the reactor fuel assemblies that define the reactor core and is heated by the fission process. In some reactor designs, the heated primary coolant is carried to a steam generator where the heated primary coolant surrenders its heat to a secondary coolant (i.e., water) disposed in the steam generator. The primary coolant then returns to the reactor core. A portion of the water that receives the heat of the primary coolant vaporizes to steam, which travels to a turbine-generator set to generate electricity. The steam that has passed through the turbine-generator set flows to a condenser that condenses the steam to water, which is then returned to the steam generator.
A type of nuclear fission reactor capable of safely generating electricity is a pool-type liquid sodium fast breeder reactor. In this regard, uranium-238 may be used as a fertile material. The uranium-238 absorbs neutrons and transmutes to fissionable plutonium-239 by means of beta decay. When plutonium-239 in turn absorbs a neutron, fission occurs to produce heat. In a fast breeder reactor, moderating materials, such as water, may not be desired as coolant. Rather, in such a pool-type liquid sodium fast breeder nuclear reactor, sodium is the coolant of choice because sodium does not significantly thermalize neutrons. Also, due to the heat transfer characteristics of sodium, the reactor core can operate at higher power densities so that size of the reactor may be reduced. In addition, sodium melts at about 100° C. (about 212° F.) and boils at about 900° C. (about 1650° F.). Thus, sodium can be used at high temperatures without boiling, thereby allowing high temperature and high pressure steam to be generated. This in turn provides increased power plant thermal efficiency.
However, the sodium coolant circulating through the reactor core becomes radioactive due to absorption of neutrons. Due to this radioactivity, reactor designers utilize intermediate heat exchange loops between the primary sodium coolant loop(s) and the steam generation loop. This lowers the of risk radioactive contamination of the turbine generator. In addition, steam generator pipe leaks may occur. If a leak were to occur in the piping carrying the sodium through the steam generator, the hot radioactive sodium passing through the steam generator will vigorously chemically react with the water and steam in the steam generator. This would radioactively contaminate the water and steam in the steam generator, thereby increasing risk of radioactive contamination of the surrounding biosphere. For all the reasons hereinabove, reactor designers incorporate use of an intermediate heat exchanger between the reactor core and the steam generator to avoid direct contact of the sodium in the core with the steam generator or turbine generator.
Thus, in the pool-type liquid sodium fast breeder nuclear reactor mentioned hereinabove, the intermediate heat exchanger forms a boundary between radioactive primary sodium in the reactor pool and nonradioactive secondary sodium in the steam generator. In other words, the intermediate heat exchanger, which is disposed in the pool of liquid sodium together with the reactor core, is typically used to remove heat from the fast breeder reactor core and transfer that heat to the external steam generator.
Attempts have been made to provide adequate removal of heat from a fast fission nuclear reactor core by use of intermediate heat exchangers. U.S. Pat. No. 4,294,658, issued Oct. 13, 1981 in the names of Peter Humphreys et al. and titled “Nuclear Reactors” discloses an intermediate heat exchange module comprising a tube-in-shell intermediate exchanger and an electromagnetic flow coupler disposed in the base region of the module for driving primary coolant through the heat exchanger. This patent addresses severe thermal shock occasioned to an intermediate heat exchanger when there is an interruption in the flow of coolant in the relevant secondary coolant circuit, for example, as caused by a failure of the secondary coolant pump. According to this patent, an object of the invention is to reduce the thermal shock occasioned to the intermediate heat exchanger of a liquid metal cooled nuclear reactor of the pool kind in such an emergency wherein there is an interruption in flow in the secondary coolant circuit.
Another attempt to provide adequate removal of heat from a fast fission nuclear reactor core by use of intermediate heat exchangers is disclosed in U.S. Pat. No. 4,324,617, issued Apr. 13, 1982 in the names of Michael G. Sowers et al. and titled “Intermediate Heat Exchanger For A Liquid Metal Cooled Nuclear Reactor And Method.” This patent discloses a heat exchanger that is used in a multi-pool, liquid metal cooled, nuclear reactor. This patent addresses accommodating differential thermal expansion between the structural components of the heat exchanger. According to this patent, the shell of the heat exchanger is heated to a temperature substantially greater than the temperature of the tubes in the heat exchanger by thermal communication with the hot pool and tensioning said tubes during operation by said heating of the shell and thereby accommodating differential thermal expansion in the heat exchanger.
Although the art recited hereinabove may disclose devices and methods that adequately serve their intended purposes, none of the art recited hereinabove appears to disclose a heat exchanger, methods therefor and a nuclear fission reactor system, as described and claimed herein.